Recently, in order to improve realistic sensation and image realism, the development of a spatial image display device for spatially forming an image has proposed. A two-dimensional display system and a three-dimensional display system are available for spatial image display. A typical system for two-dimensional display is a head mounted display (HMD), which is a system to spatially form an image displayed on a display device in front of a viewer, and to observe its virtual images. In contrast, a three-dimensional display system is generally referred to as stereoscopic display or 3D display, and provides a virtual image displayed on a two-dimensional display with the appearance of depth.
The typical three-dimensional display system is an electronic hologram system. The electronic hologram system is a system in which an interference fringe is generated by the overlapping of a reference light, such as laser, with an object light from an object to be a subject for recording, and the interference fringe is recorded. A three-dimensional image is then reproduced by irradiating the reference light onto the interference fringe when displaying the image. This electronic hologram system is a three-dimensional display system expected as a stereoscopic display system, which is natural to the human visual function. However, the technical hurdles to be overcome are high and it has not been practically used yet as an information processing device.
Similar to the electronic hologram system, another three-dimensional display system referred to as a virtual reality system is also available. This virtual reality system is a system where separate two-dimensional display devices are placed in front of the left and right eyes of a viewer, respectively. The images, corrected to form a three-dimensional image, are then displayed by the two-dimensional display devices (See, e.g., Japanese Patent Application Publication H10-319240).
However, the virtual reality system is a three-dimensional display system using binocular parallax, and it is necessary to use a display means or deflection spectacles etc. In the display means, a display unit, which shows different images in front of left and right eyes of the viewer, is positioned on a face of the viewer. Further, in the virtual reality display system, focusing adjustment to a three-dimensional image is not conducted, and a distance to a point of convergence and a distance of the focal point from eyes of an observer do not necessarily match. In other words, since the angle of convergence to determine the depth of the three-dimensional image may not correspond to a three-dimensional image position, an unnatural three-dimensional image can be displayed, causing eye strains and/or discomfort to the viewers.